1. Field of the Invention
This invention relates to a blood vessel imaging system for imaging blood vessels, and more particularly to a blood vessel imaging system which can image blood vessels with arteries and veins distinguished from each other. This invention also relates to a method and system for determining a spatial distribution of a pulsation wave signal representing a pulsation wave of an artery of an organism.
2. Description of the Related Art
In the clinical field, there has been a wide demand for imaging arteries and veins clearly distinguished from each other. For example, since arteriosclerosis generally starts at the periphery of the arteries, it will be useful in diagnosing arteriosclerosis if the inner walls of the peripheral arteries can be imaged distinguished from those of the veins.
There has been wide known angiography as a system for imaging blood vessels. However angiography is disadvantageous in that load on the testee is heavy and the testee generally must stay in the hospital.
Further there has been proposed technique for imaging a part of an organism on the basis of penetration of light through the part as disclosed in xe2x80x9cJapanese ME Academy Magazine BMExe2x80x9d, vol.8, No.5, 1994, pp. 41xcx9c50. However it is very difficult to image arteries and veins clearly distinguished from each other by the technique.
Also, as a technique for extracting information representing a pulsation wave of an artery, a technique has heretofore been known, in which measuring light beams having two different wavelengths are irradiated to an organism, a logarithm of pulsation wave amplitude is calculated from each of detection signals obtained by detecting the measuring light beams having passed through the organism, and thereafter pulsation wave components are calculated in accordance with the ratio of the two logarithms to each other. However, with this technique for directly detecting the measuring light beams, it is impossible to determine a spatial distribution of a pulsation wave signal representing pulsation wave information.
In view of the foregoing observations and description, the primary object of the present invention is to provide a blood vessel imaging system which can image blood vessels with arteries and veins clearly distinguished from each other without exposing the testee to heavy load.
Another object of the present invention is to provide a method of determining a spatial distribution of a pulsation wave signal, wherein a spatial distribution of a pulsation wave signal representing a pulsation wave of an artery of an organism is capable of being determined.
The specific object of the present invention is to provide a system for carrying out the method of determining a spatial distribution of a pulsation wave signal.
In blood vessel imaging systems in accordance with one aspect of the present invention, an optical heterodyne detection system is employed in order to ensure high spatial resolution to an organism as a scattering medium, and arteries and veins are distinguished from each other on the basis of the fact that the output signal of the heterodyne detection system is modulated by the pulsation wave unique to arteries when the measuring light beam is projected onto an artery.
That is, in accordance with a first aspect of the present invention, there is provided a blood vessel imaging system comprising
a measuring light source which emits a measuring light beam,
a scanning means which causes the measuring light beam to scan an organism,
an optical heterodyne detection system consisting of an optical system which splits the measuring light beam upstream of the organism into a first light beam traveling to impinge upon the organism and a second light beam traveling not to impinge upon the organism and combines the second light beam with the first beam emanating from the organism into a combined light beam, a frequency shifter which causes the first and second light beams to have frequencies different from each other, and a beat component detecting means which detects beat components of the combined light beam, and
an image signal generating means which generates an image signal on the basis of the ratio of the intensity of a pulsation wave band signal to the intensity of a beat signal included in an output signal of the optical heterodyne detection system.
It is preferred that the blood vessel imaging system be further provided with a frequency analysis means which analyzes the output signal of the optical heterodyne detection system, and the image signal generating means obtains the intensity ratio on the basis of the pulsation wave band signal and the beat signal separated from each other by the frequency analysis means on a frequency axis.
It is preferred that the image signal generating means generates an image signal representing artery parts of the organism when the intensity ratio is higher than a predetermined threshold level.
A blood vessel imaging system in accordance with a second embodiment of the present invention comprises a measuring light source, a scanning means and an optical heterodyne detection system similar to those in the blood vessel imaging system of the first aspect and is further provided with an image signal generating means which generates an image signal on the basis of the degree of modulation at a pulsation wave band frequency of a beat signal included in an output signal of the optical heterodyne detection system.
In the blood vessel imaging system in accordance with the second aspect, it is preferred that a pulsation wave detecting means which detects a pulsation wave of the organism be provided, and the image signal generating means samples the signal value when the beat signal is in a predetermined phase on the basis of an output signal of the pulsation wave detecting means.
Further it is preferred that the image signal generating means generates an image signal representing artery parts of the organism when the degree of modulation is higher than a predetermined threshold level.
Further it is preferred in the blood vessel imaging systems in accordance with both the first and second aspects of the present invention that the measuring light source comprises a linear or two-dimensional array of a plurality of light emitting portions, and the optical heterodyne detection system is arranged to be able to detect in parallel beat components of the combined light beams based on the measuring light beams from the respective light emitting portions, and the measuring light source and the optical heterodyne detection system also function as at least a part of said scanning means.
The beat component detection signal (beat signal) output from the heterodyne detection system described above represents intensity of only straight light components traveling straight through the organism or scattered light components close to the straight light components except influence of scattering by the organism which is a scattering medium.
The artery part and the vein part are distinguished from each other in the following manner. While the first light beam split from the measuring light beam as emitted from the light source is being projected onto an artery part, the output signal of the optical heterodyne detection system consists of a pulsation wave signal a at a frequency of about 1 Hz generated by pulsation of the artery and a beat signal b superimposed one on the other as shown in FIG. 5. While the first light beam is being projected onto a vein part, there is generated no pulsation wave signal.
When the output signal which varies with time as shown in FIG. 5 is sampled at a certain timing and subjected to frequency analysis, a spectrum such as shown in FIG. 2 is obtained. In FIG. 2, the pulsation wave signal component is indicated at A and the beat signal component is indicated at B. The intensities of the pulsation wave signal and the beat signal vary in response to the pulsation as shown by the solid line and the broken line in FIG. 2 and with attenuation of the first light beam due to absorption and/or scattering by the organism.
Though the intensities of the pulsation wave signal and the beat signal vary, the ratio of the intensity of the former to that of the latter is kept unchanged. Accordingly when the intensity ratio is higher than a certain level, it may be considered that a pulsation wave signal is being generated, that is, the first light beam is being projected onto an artery part. When the output signal of the optical heterodyne detection system is space-divided as the measuring light beam (the first light beam) scans the organism and an image signal component is generated for each scanning spot, the artery part can be imaged on the basis of an image signal made up of a plurality of image signal components thus obtained.
For example, when the image signal generating means generates an image signal component bearing thereon a relatively high density (low brightness) when the intensity ratio is higher than a predetermined threshold value and generates an image signal component bearing thereon a relatively low density (high brightness) when the intensity ratio is not higher than the threshold value, an image in which the artery part is shown as a relatively high density part on a background at a relatively low density can be obtained.
When the image signal generating means is arranged to generate an image signal component bearing thereon a density which becomes higher as the intensity ratio increases, an image in which the artery part can be clearly distinguished from other parts as a relatively high density part can also be obtained.
When the beat signal b is extracted, for instance, by passing a signal, whose waveform is as shown in FIG. 5, through a band-pass filter, change with time of the intensity of the extracted beat signal b is as shown in FIG. 4. As shown in FIG. 4, the intensity of the beat signal b periodically changes since the beat signal b is modulated by the pulsation signal.
The degree of modulation of the beat signal b represented by formula {IF(H)xe2x88x92IF(L)}/{IF(H)+IF(L)}, wherein IF(H) represents the peak intensity of the beat signal b and IF(L) represents the bottom intensity of the same, is basically kept unchanged even if the intensities of the pulsation wave signal and the beat signal vary with attenuation of the first light beam due to absorption and/or scattering by the organism. Accordingly when the degree of modulation is higher than a certain level, it may be considered that the first light beam is being projected onto an artery part.
When the output signal of the optical heterodyne detection system is space-divided as the measuring light beam (the first light beam) scans the organism and an image signal component is generated for each scanning spot, the artery part can be imaged on the basis of an image signal made up of a plurality of image signal components thus obtained.
For example, when the image signal generating means generates an image signal component bearing thereon a relatively high density (low brightness) when the degree of modulation is higher than a predetermined threshold value and generates an image signal component bearing thereon a relatively low density (high brightness) when the degree of modulation is not higher than the threshold value, an image in which the artery part is shown as a relatively high density part on a background at a relatively low density can be obtained.
When the image signal generating means is arranged to generate an image signal component bearing thereon a density which becomes higher as the degree of modulation increases, an image in which the artery part can be clearly distinguished from other parts as a relatively high density part can also be obtained.
When there is provided a pulsation wave detecting means which detects a pulsation wave of the organism, and the image signal generating means samples the signal value when the beat signal is in a predetermined phase on the basis of an output signal of the pulsation wave detecting means, it is possible to accurately sample the peak intensity IF(H) and the bottom intensity IF(L) of the beat signal 6, whereby an accurate value of the degree of modulation can be constantly obtained.
Further when a measuring light source comprising a linear or two-dimensional array of a plurality of light emitting portions and an optical heterodyne detection system which can detect in parallel beat components of the combined light beams based on the measuring light beams from the respective light emitting portions are employed so that the measuring light source and the optical heterodyne detection system also function as at least a part of said scanning means, it becomes unnecessary for the scanning means to mechanically cause the measuring light beam to scan the organism in at least one direction, whereby the scanning speed, which results in the imaging speed, can be increased.
This is especially advantageous in the case where the image signal generating means samples the signal value when the beat signal is in a predetermined phase and sampling of the signal requires a relatively long time.
In blood vessel imaging systems in accordance with another aspect of the present invention, a pair of optical heterodyne detection systems which are different in frequency of measuring light beam are employed in order to ensure high spatial resolution to an organism as a scattering medium, the degree of oxygen saturation of the part of the organism onto which the measuring light beams are projected is determined on the basis of the output signals of the optical heterodyne detection systems, and arteries and veins are distinguished from each other on the basis of the fact that the degree of oxygen saturation is higher in arteries than in veins.
In accordance with a third aspect of the present invention, there is provided a blood vessel imaging system comprising
a measuring light source which emits first and second measuring light beams which are different in frequency,
an incident optical system which causes the first and second measuring light beams to impinge upon the organism in the same position,
a scanning means which causes the first and second measuring light beams to scan an organism,
a first optical heterodyne detection system consisting of an optical system which splits the first measuring light beam upstream of the organism into a first section traveling to impinge upon the organism and a second section traveling not to impinge upon the organism and combines the second section with the first section emanating from the organism into a first combined light beam, a frequency shifter which causes the first and second sections of the first light beam to have frequencies different from each other, and a beat component detecting means which detects beat components of the first combined light beam,
a second optical heterodyne detection system consisting of an optical system which splits the second measuring light beam upstream of the organism into a first section traveling to impinge upon the organism and a second section traveling not to impinge upon the organism and combines the second section with the first section emanating from the organism into a second combined light beam, a frequency shifter which causes the first and second sections of the second light beam to have frequencies different from each other, and a beat component detecting means which detects beat components of the second combined light beam, and
an image signal generating means which calculates characteristic value in one-to-one correspondence to a degree of oxygen saturation on the basis of beat component detection signals respectively output from the first and second optical heterodyne detection systems, and generates an image signal on the basis of the characteristic value.
In this specification, the characteristic value may be the degree of oxygen saturation itself.
For example, the image signal generating means may generate an image signal employing, as the characteristic value, the ratio between a logarithm of amplitude of the beat component detection signal output from the first optical heterodyne detection system due to a pulsation wave of the organism and a logarithm of amplitude of the beat component detection signal output from the second optical heterodyne detection system due to the pulsation wave of the organism.
In the case where the image signal is generated on the basis of the ratio of the logarithms of the amplitudes, for instance, there are provided a filtering means which extracts modulated components at frequencies in the pulsation wave bands out of the beat component detection signals respectively output from the first and second optical heterodyne detection systems, and a level meter which measures levels of the signals extracted by the filtering means, and the image signal generating means obtains the amplitudes due to the pulsation wave on the basis of output signals of the level meter.
Otherwise, a pulsation wave detecting means which detects a pulsation wave output from the organism and a sampling means which samples the beat component detection signals respectively output from the first and second optical heterodyne detection systems at timings at which the beat component detection signals are maximized and minimized on the basis of an output signal of the pulsation wave detecting means may be provided, and the image signal generating means may obtain the amplitudes due to the pulsation wave on the basis of the sampled values of beat components detection signals.
It is preferred that the first and second measuring light beams respectively be 760 nm and 930 nm in frequency.
Further it is preferred that the image signal generating means outputs an image signal representing an artery part of the organism when it calculates said characteristic value to be a value corresponding to a degree of oxygen saturation of 80 to 90%.
Further it is preferred in the blood vessel imaging system in accordance with the third aspect of the present invention that the measuring light source comprises first and second linear or two-dimensional arrays of a plurality of light emitting portions, the light emitting portions of the first array emitting a plurality of first measuring light beams and the light emitting portions of the second array emitting a plurality of second measuring light beams, the first optical heterodyne detection system is arranged to be able to detect in parallel beat components of the first combined light beams based on the first measuring light beams from the respective light emitting portions, the second optical heterodyne detection system is arranged to be able to detect in parallel beat components of the second combined light beams based on the second measuring light beams from the respective light emitting portions, and the measuring light source and the optical heterodyne detection systems also function as at least a part of said scanning means.
The beat component detection signal (beat signal) output from each of the first and second heterodyne detection systems described above represents intensity of only straight light components traveling straight through the organism or scattered light components close to the straight light components except influence of scattering by the organism which is a scattering medium. The beat signals are modulated at a frequency of about 1 Hz by the pulsation wave of the organism when the first and second measuring light beams are being projected onto an artery part of the organism.
By using the first and second measuring light beams which are different in frequency and calculating, for instance, the ratio between a logarithm of amplitude of the beat component detection signal output from the first optical heterodyne detection system due to a pulsation wave of the organism and a logarithm of amplitude of the beat component detection signal output from the second optical heterodyne detection system due to the pulsation wave of the organism, the degree of oxygen saturation of blood in the part of the organism exposed to the measuring light beams can be obtained. Since the degree of oxygen saturation of arterial blood is 80 to 90%, an image in which the artery part can be clearly distinguished from vein parts and other can be obtained by generating an image signal representing artery parts when a degree of oxygen saturation of 80 to 90% is detected.
The intensity of the beat signal with attenuation of the measuring light beam due to absorption and/or scattering by a tissue of the organism and the attenuation of the measuring light beam changes with the thickness of the tissue and the like. Though the intensities of the beat signals vary, the aforesaid ratio of the logarithms is constantly in one-to-one correspondence to the degree of oxygen saturation with the change in the intensities of the beat signals compensated for, whereby the artery parts can be accurately imaged.
For example, when the image signal generating means generates an image signal component bearing thereon a relatively high density (low brightness) when the degree of oxygen saturation represented by the characteristic value is 80 to 90% and generates an image signal component bearing thereon a relatively low density (high brightness) when the degree of oxygen saturation is lower than 80%, an image in which the artery part is shown as a relatively high density part on a background at a relatively low density can be obtained.
Further when a measuring light source comprising first and second linear or two-dimensional arrays of a plurality of light emitting portions, the light emitting portions of the first array emitting a plurality of first measuring light beams and the light emitting portions of the second array emitting a plurality of second measuring light beams, a first optical heterodyne detection system arranged to be able to detect in parallel beat components of the first combined light beams based on the first measuring light beams from the respective light emitting portions, and a second optical heterodyne detection system arranged to be able to detect in parallel beat components of the second combined light beams based on the second measuring light beams from the respective light emitting portions are employed so that the measuring light source and the optical heterodyne detection systems also function as at least a part of said scanning means, it becomes unnecessary for the scanning means to mechanically cause the measuring light beam to scan the organism in at least one direction, whereby the scanning speed, which results in the imaging speed, can be increased.
This is especially advantageous in the case where the image signal generating means obtains the characteristic value from the values of the beat signals when the beat signals are in a predetermined phase on the basis of the output signal of the pulsation wave detecting means and, accordingly, sampling of the signal requires a relatively long time.
In accordance with a fourth aspect of the present invention, there is provided a system for determining a spatial distribution of a pulsation wave signal, comprising
a measuring light source which emits a measuring light beam impinging upon an organism,
an optical heterodyne detection system consisting of an optical system which splits the measuring light beam upstream of the organism into a first light beam traveling to impinge upon the organism and a second light beam traveling not to impinge upon the organism and combines the second light beam with the first beam emanating from the organism into a combined light beam, a frequency shifter which causes the first and second light beams to have frequencies different from each other, and a beat component detecting means which detects beat components of the combined light beam, and
a pulsation wave signal generating means for generating a pulsation wave signal, which represents a pulsation wave of the organism, on the basis of an output signal of the optical heterodyne detection system.
By way of example, the pulsation wave signal generating means may be a means, which generates the pulsation wave signal on the basis of the ratio of the intensity of a pulsation wave band signal to the intensity of a beat signal included in the output signal of the optical heterodyne detection system. In such cases, it is preferred that the pulsation wave signal generating means generates a pulsation wave signal, which represents a pulsation wave of an artery of the organism, when the intensity ratio is higher than a predetermined threshold level.
Alternatively, the pulsation wave signal generating means may be a means, which generates the pulsation wave signal on the basis of the degree of modulation at a pulsation wave band frequency of the beat signal included in the output signal of the optical heterodyne detection system. In such cases, it is preferred that the pulsation wave signal generating means generates a pulsation wave signal representing a pulsation wave of an artery of the organism when the degree of modulation is higher than a predetermined threshold level.
In accordance with a fifth aspect of the present invention, there is provided a system for determining a spatial distribution of a pulsation wave signal, comprising
a measuring light source which emits first and second measuring light beams which are different in frequency,
an incident optical system which causes the first and second measuring light beams to impinge upon the organism in the same position,
a scanning means which causes the first and second measuring light beams to scan an organism,
a first optical heterodyne detection system consisting of an optical system which splits the first measuring light beam upstream-of the organism into a first section traveling to impinge upon the organism and a second section traveling not to impinge upon the organism and combines the second section with the first section emanating from the organism into a first combined light beam, a frequency shifter which causes the first and second sections of the first light beam to have frequencies different from each other, and a beat component detecting means which detects beat components of the first combined light beam,
a second optical heterodyne detection system consisting of an optical system which splits the second measuring light beam upstream of the organism into a first section traveling to impinge upon the organism and a second section traveling not to impinge upon the organism and combines the second section with the first section emanating from the organism into a second combined light beam, a frequency shifter which causes the first and second sections of the second light beam to have frequencies different from each other, and a beat component detecting means which detects beat components of the second combined light beam, and
a pulsation wave signal generating means which calculates characteristic value in one-to-one correspondence to a degree of oxygen saturation on the basis of beat component detection signals respectively output from the first and second optical heterodyne detection systems, and generates a pulsation wave signal on the basis of the characteristic value.
In the system for determining a spatial distribution of a pulsation wave signal in accordance with the fifth aspect of the present invention, the pulsation wave signal generating means should preferably generate the pulsation wave signal by employing, as the characteristic value, the ratio between a logarithm of amplitude of the beat component detection signal output from the first optical heterodyne detection system due to a pulsation wave of the organism and a logarithm of amplitude of the beat component detection signal output from the second optical heterodyne detection system due to the pulsation wave of the organism.
Also, in the system for determining a spatial distribution of a pulsation wave signal in accordance with the fifth aspect of the present invention, a wavelength xcex1 of the first measuring light beam should preferably fall within the range of 600 nm less than xcex1 less than 805 nm, and a wavelength xcex2 of the second measuring light beam should preferably fall within the range of 805 nm less than xcex2 less than 1, 100 nm. The wavelength xcex1 of the first measuring light beam should more preferably be 760 nm, and the wavelength xcex2 of the second measuring light beam should more preferably be 930 nm.
The present invention further provides a method of determining a spatial distribution of a pulsation wave signal, comprising the steps of obtaining a pulsation wave signal by utilizing the system for determining a spatial distribution of a pulsation wave signal in accordance with the fourth or fifth aspect of the present invention, i.e. the system utilizing the optical heterodyne detection system.
With the systems for determining a spatial distribution of a pulsation wave signal in accordance with the present invention, wherein the pulsation wave signal is obtained by utilizing the optical heterodyne detection technique, basically, the pulsation wave signal corresponding to only the part, upon which the measuring light beam impinges, can be obtained. Therefore, the spatial distribution of the pulsation wave signal concerning the organism can be determined by obtaining pulsation wave signals corresponding to two or more different points on the organism by use of the system for determining a spatial distribution of a pulsation wave signal.